Bianchi, Luciana and Hutchings, John B. and Efremova, Boryana and Herald, James E. and Bressan, Alessandro and Martin, Christopher (2009) Ultraviolet quasi-stellar objects. Astronomical Journal, 137 (4). pp. 3761-3777. ISSN 0004-6256 http://resolver.caltech.edu/CaltechAUTHORS:20090420-101134109
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We present a sample of spectroscopically confirmed quasi-stellar objects (QSOs) with FUV–NUV color (as measured by Galaxy Evolution Explorer (GALEX) photometry, FUV band: 1344–1786 Å, NUV band: 1771–2831 Å) bluer than canonical QSO templates and than the majority of known QSOs. We analyze their FUV to NIR colors, luminosities, and optical spectra. The sample includes a group of 150 objects at low redshift (z < 0.5), and a group of 21 objects with redshift 1.7 < z < 2.6. For the low-redshift objects, the “blue” FUV–NUV color may be caused by enhanced Lyα emission, since Lyα transits the GALEX FUV band from z = 0.1 to z = 0.47. Synthetic QSO templates constructed with Lyα up to three times stronger than in standard templates match the observed UV colors of our low-redshift sample. Optical photometric and spectroscopic properties of these QSOs are not atypical. The Hα emission increases, and the optical spectra become bluer, with increasing absolute UV luminosity. The lack of selected objects at intermediate redshift is consistent with the fact that for z = 0.48–1.63, Lyα is included in the GALEX NUV band, making the observed FUV–NUV redder than the limit of our sample selection. The UV-blue QSOs at redshift ~2, where the GALEX bands sample rest-frame ≈450– 590 Å (FUV) and ≈590–940 Å (NUV), are fainter than the average of UV-normal QSOs at similar redshift in NUV, while they have comparable luminosities in other bands. Therefore, we speculate that their observed FUV–NUV color may be explained by a combination of steep flux rise toward short wavelengths and dust absorption below the Lyman limit, such as from small grains or crystalline carbon (nanodiamonds). The ratio of Lyα to C iv could be measured in 10 objects; it is higher (30% on average) than for UV-normal QSOs, and close to the value expected for shock or collisional ionization. However, optical spectra are taken at different times than the UV photometry, which may bias the comparison if lines are variable. These QSO groups are uniquely set apart by the GALEX photometry within larger samples, given that their optical properties are not unusual.
|Additional Information:||©2009. American Astronomical Society. Received 2008 May 20, accepted for publication 2008 December 27. Published 2009 March 4. We are very grateful to Vahram Chavushyan, Lucio Buson, and Sebastien Heinis for discussions at different stages of this work, and to the anonymous referee for many comments which led to useful clarifications and improved the paper. More information and related papers are available at the author’sWeb site at http://dolomiti.pha.jhu.edu. GALEX is a NASA Small Explorer, launched in 2003 April. We gratefully acknowledge NASA’s support for construction, operation, and science analysis of the GALEX mission, developed in cooperation with the Centre National d’Etudes Spatiales of France and the Korean Ministry of Science and Technology. The data presented in this paper were obtained from the Multimission Archive at the Space Telescope Science Institute (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NAG5-7584 and by other grants and contracts.|
|Subject Keywords:||quasars: absorption lines – quasars: emission lines – quasars: general – ultraviolet: galaxies|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Marie Noren|
|Deposited On:||21 Apr 2009 17:51|
|Last Modified:||26 Dec 2012 10:58|
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